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The Singhbhum Shear Zone, eastern India is known for its large uranium and copper resources. Uraninite, the most abundant U-ore mineral, occurs as inclusions in magnetite, ilmenite, apatite, tourmaline, biotite, chlorite and allanite. Shared grain boundaries of uraninite with magnetite and ilmenite are common. Uraninite also shows complex symplectic intergrowth with ilmenite. In some places, the symplectite shares a straight grain boundary with ilmenite and magnetite and is surrounded and enveloped by magnetite. The symplectite could be formed by the reaction of separate U-bearing and Fe–Ti-bearing minerals or by the breakdown of a single U–Ti–Fe-bearing mineral under changed physicochemical conditions. Using the textural settings, geochemistry, and known occurrence of brannerite (UTi2O6) in the shear zone, we propose that the uraninite–ilmenite symplectite is most likely formed by the breakdown of brannerite by interaction with a Fe-bearing hydrothermal fluid. Based on in-situ EPMA chemical dating of uraninite in the symplectite and the known ages of uraninite formation/recrystallization in the shear zone, we further suggest that the symplectite formed during 950±50 Ma, an age which represents a pervasive tectonothermal event in the shear zone. [ABSTRACT FROM AUTHOR]

The Nagercoil block, which is situated at the southernmost part of the Southern Granulite Terrane of south India, is dominantly comprised of garnet-bearing I-type massive charnockites. These charnockites preserve imprints of two major thermal events timed at Palaeoproterozoic (∼2.0–1.9 Ga) and Neoproterozoic (∼550 Ma). However, there is a lack of understanding on correlating the growth and stabilization of garnets within these charnockites, corresponding to these events. In the present study, rare earth element-based partition modeling between garnet and age-constrained accessory phases such as zircon and monazite is carried out to identify the exact timing of garnet formation in a representative sample from the area. U-Pb dating of zircon cores constrains the timing of protolith emplacement at ∼2.0 Ga and metamorphism at ∼550 Ma from zircon rims and monazites. REE modeling of zircon and monazite from these age domains suggests an equilibrium relation between the Neoproterozoic zircon rims and monazites, while the Palaeoproterozoic zircon cores are not in equilibrium with garnet. These results suggest that the formation of garnets in the Nagercoil charnockites is in response to the Neoproterozoic metamorphism recorded in the terrane associated with the final stage of the Gondwana supercontinent assembly. [ABSTRACT FROM AUTHOR]

The transition from magmatic to solid-state deformation in granitic rocks is observed either in cases of shear-zone-controlled magma emplacement or in cases of magma-assisted shear-zone nucleation. In the latter case, pre-emplaced granitoid magmas that are later subjected to a dynamic regime with decreasing temperature accommodate stress that causes strain localization. Shear zones in this case are generally restricted to the contact zone between wall rock and granitoid magma, as it represents large viscosity discontinuities. In large granitic provinces (LGPs), however, rheological properties are mainly driven by the changes in physical parameters of granitic magmas, and thus, shear-zone nucleation in magmatic or sub-magmatic conditions cannot be attributed to the viscosity discontinuities caused by the presence of wall rock. Rather, in LGPs, it is only dependent on the rheological discontinuities within the granitic magmas, and thus, a detailed knowledge of the transition from magmatic to solid-state deformation can give important insights into the dynamic history of granitoid magmas in LGPs. The ~2.5 Ga old, voluminous granitoids of Bundelkhand Craton have been affected by multiple shear systems. Among these, the NE–SW and E–W shear systems are major systems that affected the granitoid microstructures. The NE–SW shear system transects the craton as multiple small- and large-scale shear zones, while the E–W shear system is mainly present in the central part of the Bundelkhand Craton. A petrographic study of a ~25 km long, NE–SW trending Baldeogarh shear zone and granite microstructures in associated areas show important magmatic and solid-state deformation features. Overprinting of solid-state deformation over sub-magmatic deformation confirms the progression of deformation with decreasing temperature. Contrary to this, E–W trending microstructures are primarily magmatic to sub-magmatic with rare small-scale solid-state deformation. This indicates that the E–W shear system is older in southern Bundelkhand and was followed by the NE–SW shear system in magmatic conditions. Therefore, the presence of a transition from magmatic to solid-state deformation along the NE–SW trending shear zone indicates that bulk cooling of the granitoid took place while the NE–SW shear system was active. Research Highlights: Transition from magmatic to solid-state deformation in Large Granitic Provinces. Magmatic to solid-state deformation along NE-SW shear zone. Overprinting textures of transition from sub-magmatic to solid-state deformation. [ABSTRACT FROM AUTHOR]

Granitic magmatism is considered an essential mechanism for crustal recycling in the rapidly accreted crust. Mode of emplacement and ascent of granitic magma, together with the exhumation of granites, especially along plate margins, hold a vital clue to the Earth's thermomechanical workings. The present study investigates the role of ductile deformation in the exhumation of the granitic rocks of the Karakoram batholith (KB), north of the Shyok Suture Zone (SSZ), NW Trans-Himalaya. Textural and thermobarometric studies suggest that crystallization of the KB initiated at ~ 700–800 °C and ~ 4.2–7.5 kb. Microstructures of feldspar and quartz pertaining to temperatures > 700 °C together with the late crystallization of muscovite evince submagmatic flow. Muscovite-rich domains typically exhibit microstructures indicating temperatures < 650 °C. Aligned mica, along with penetrative grain boundary migration and moderately strong crystallographic preferred orientation in quartz, represents the highest deformation intensity that probably prevailed during the initial stages of collision along the SSZ. On the other hand, randomly oriented muscovites that crystallized following peak deformation intensity occur exclusively in micro-domains with polygonal quartz grains. Phase transformation of micas to chlorite occurred at ~ 280–400 °C, during which deformation progressed by minor GBM at relatively slower rates. This study implies that submagmatic flow followed by subsolidus solid-state ductile deformation was significant in the exhumation of the granites from a depth of ~ 19–28 to ~ 5.5–9.5 km. Temperature estimates, coupled with geochronological data, yield average cooling rates of ~ 11–18 °C/Ma from ~ 110–85 Ma that gradually decreased to ~ 1.8 °C/Ma after ~ 85 Ma. [ABSTRACT FROM AUTHOR]

In a geochemical and geochronological investigation of Archean and Proterozoic magmatism in the Nellore Schist Belt, we conducted SHRIMP U-Pb analyses of zircons from two cospatial granitic bodies at Guramkonda and Vendodu. The former is a Ba- and Sr-rich hornblende-bearing tonalite, whereas the latter is a Rb-, Zr-, Pb-, Th-, U-, and REE-rich biotite-bearing leucogranite. The Guramkonda tonalite displays a restitic texture with remnants of trapped granitic melt, whereas the Vendodu leucogranite contains residual/partially melted plagioclase grains. Both rock types contain two generations of zircon: tonalite contains a group of euhedral zoned zircons enclosed within plagioclase and a group of subhedral patchy zircons associated with trapped melt (quartz + feldspar matrix), and leucogranite also contains a group of doubly terminated euhedral zircons included within orthoclase as well as a group of zircons with visible cores mantled by later rim growth. Cathodoluminescence images also clearly document two distinctly textured varieties of zircon: the tonalite contains a population characterized by narrowly spaced uninterrupted oscillatory zoning and a second population lacking zoning but exhibiting a random distribution of dark (U-rich) and light (U-poor) regions; the leucogranite contains U-rich zoned zircons and U-poor zircon cores mantled by U-rich rims. The REE chemistry of zircon cores from the Vendodu leucogranite is very similar to the REE of zoned zircons from the Guramkonda tonalite. Zircon ages from both plutons exhibit bimodal distributions in U-Pb concordia diagrams. The tonalite defines an age of 2,521 Ma ± 5 Ma for zoned magmatic zircons and 2,485 Ma ± 5 Ma for unzoned newly precipitated zircons, whereas the leucogranite has an age of 2,518 Ma ± 5 Ma for U-poor zircon cores (relics of the tonalite pluton) and 2,483 Ma ± 3 Ma for U-rich zoned magmatic zircons. The trace element geochemistry of the ~2,520 Ma zircons is distinctly different from the ~2,485 Ma zircons, irrespective of the host rock. Our textural, CL image, and SHRIMP U-Pb analyses document the origin of the leucogranite by partial melting of the tonalite. High alkalis (NaO + KO), Rb, Nb, HREE, FeO/MgO and low Ca, Al, Ba, Sr, and large negative Eu anomalies characterize the leucogranite as a thermal minimum melt, whereas the very low K and Rb of the tonalite attests to its residual nature. We suggest that the leucogranite formed by high-T (900-950°C), moderate-pressure (<10 kbar) dehydration partial melting of the tonalite under reducing conditions. The calculated source compositions of the leucogranite melt and the tonalite residue show strong similarities to melts that are considered to have been produced in a subduction-zone environment. The leucogranite probably formed in a post-collisional realm immediately after accretion of the tonalitic crust. [ABSTRACT FROM AUTHOR]